The risk of injury has always been a problem for the athlete. Such injuries are not limited to contact sports such as football, but also frequently occur in non-contact sports such as running and jogging. Many people have become interested in jogging in recent years, as evidenced by the proliferation of community marathons and other long-distance races. Long-distance runners in particular have experienced many running-related injuries, which at the very least, interrupt their running activities and at the worst can prevent them from further engaging in serious running competition and lead to physical complications later in life. Perhaps the most serious of the running-related injuries is injury to the knee joint or the cartilage surrounding the knee joint. Such injuries are often caused by the twisting motion which accompanies the running cycle as the feet hit and push off from the ground.
Excessive pronation and supination are frequently causes of knee injuries. Pronation is the rotation of the foot from the "lateral" or outside portion thereof to the "medial" or inside portion. During the running cycle, the foot first hits the ground on the outer portion of the heel. The weight is then transferred to the forward portion to provide lift-off from the ground at the toes. Supination is the rotation of the foot from the medial to the lateral side. Rotation of the foot caused by either pronation or supination creates a twisting action on the knee, which may lead to serious injuries. Furthermore, this twisting action is exacerbated by the fact that in the conventional running shoe the initial push-off from the ground occurs along the metatarsal-phalangeal joints on the lateral side of the foot, thereby causing the foot to be directed outwardly from the desired direction of running.
Two basic approaches have been used to attempt to solve the problem of excessive pronation. One such approach involves building up the medial side of the shoe so that the medial side is slightly elevated with respect to the lateral side. Thus, during the running cycle, the elevated medial side of the shoe tends to counteract the natural rotation of the foot from the lateral side to the medial side. The medial side may be elevated either by building up the medial side of the support cushion on the inside of the shoe, or, alternatively, by elevating the medial side of the midsole of the shoe, as is shown in U.S. Pat. No. 4,180,924. The second basic approach involves providing a denser, firmer material on the medial side of the shoe to counteract pronation. For individuals having a problem with excessive supination, the shoe is built up on the lateral side.
Another problem often associated with conventional running shoes is that the shoe is too stiff and inflexible to allow the foot to properly bend for efficient lift-off from the ground. The need for a flexible show to provide for efficient bending of the foot during the running cycle must be balanced against the need for a sturdy, sufficiently cushioned shoe structure to absorb the impact of the feet hitting the ground during running.
U.S. Pat. No. 4,155,180 attempts to solve this problem by providing a running shoe in which the forward portion of the midsole and tread is much more flexible than the rear portion thereof to allow the forward portion to flex during lift-off from the ground. Although the flexibility of the shoe is somewhat enhanced, a substantial amount of force must still be exerted by the foot muscles, and in particular the foot muscles which bend the third, fourth and fifth metatarsal-phalangeal joints on the lateral side of the foot, to push off from the ground. As such, the efficiency of the runner is reduced and the foot muscles and joints on the lateral side of the foot are subject to overfatigue.
Another approach is shown in U.S. Pat. No. 4,262,435, wherein a flexure break segment of reduced thickness is provided on the solepiece along the metatarsal-phalangeal joint line. The segments are disposed along two intersecting lines, one of which extends between the first and second metatarsal-phalangeal joints and the second of which extends between the second and the fifth metatarsal-phalangeal joints. This enables each joint to flex indiviudually in accordance with the natural flexing action of the foot. Although the flexing action of the foot is somewhat enhanced by these segments of reduced thickness, the initial force needed to push off from the ground is still provided by the muscles on the lateral side of the foot, instead of the stronger muscles on the medial side thereof, resulting in a less efficient push-off from the ground and causing the foot to move outwardly from the desired running direction.